Oxidative Desulfurization of Middle-Distillate Fuels Using Activated Carbon and Power Ultrasound

The oxidative desulfurization (ODS) of jet and diesel fuels was studied using hydrogen peroxide plus formic acid as the oxidant, activated carbon as a reaction enhancer, and power ultrasound for phase dispersion. When the ODS treatment is followed by an activated alumina post-processing step, overall sulfur removal performance was 98% for JP-8 (at pH 1.4), 94% for diesel (at pH 1.5), and >88% for ultralow-sulfur diesel (at pH 1.5). The ODS treatment converts sulfur compounds to sulfones/sulfoxides, and activated alumina removes the oxidized sulfur compounds to yield a low-sulfur fuel. Control tests reveal that removal of any of the four reaction components (ultrasound, carbon, hydrogen peroxide, and formic acid) reduces the ODS removal performance, with hydrogen peroxide being the most crucial. The response of ODS removal performance to initial oxidant concentrations is consistent with performic acid, formed in situ from hydrogen peroxide and formic acid, being the active oxidizing species. Power ultrasound promotes dispersion of the three immiscible phases (fuel, water, and carbon), accelerates interfacial mass transfer, and may also accelerate the reaction via the sonochemical effect. Activated carbon increases the ODS reaction rate up to 12-fold. Rate data for different benzothiophene compounds show no evidence of steric hindrances, either in the presence or in the absence of carbon. Two types of activated carbon were tested as ODS enhancers: a phosphoric acid treated, wood-based activated carbon material and a thermally activated peat-based carbon material. Both carbon materials improved ODS reaction rates relative to the uncatalyzed control, but the wood-based carbon was superior to the peat-based carbon for all monitored sulfur compounds. The wood-based carbon had greater surface area, pore volume, and surface acidity than the peat-based carbon, making it difficult to attribute the enhancement effect to any single characteristic. Chemical analysis of the treated fuel revealed that <1% of the fuel hydrocarbon compounds was oxidized during the ODS treatment and that aromatic compounds were the most likely to be oxidized.